Observation systems usable both in daylight and at night typically include a visible-light telescope and an infrared imaging device. Typically, these devices have separate optical paths. Reticle projections in these paths have to be carefully aligned for accurate boresighting, and this alignment is difficult to maintain in a vehicular or aeronautical environment where the equipment is subjected to severe vibration. In addition, size and weight of this type of system is often a critical factor, and it is therefore desirable for the optical path to be as compact as possible.
Prior art in the observation systems field includes: U.S. Pat. No. 3,752,587 which deals with a system for boresighting a combined infrared laser and low-light telescope; U.S. Pat. No. 4,139,769 which deals with a similar system; U.S. Pat. No. 4,385,834 which describes boresighting by aligning two laser beams; U.S. Pat. No. 4,569,591 which relates to a boresight mechanism for a laser/FLIR rangefinder; and U.S. Pat. No. 4,653,911 which discloses a two-path system for forming two images of a reticle for coarse and fine boresighting.